support new expires_in field for tokens
https://community.fitbit.com/t5/Web-API/OAuth-2-0-expiresIn-value/m-p/1461298/highlight/true#M5862
Should this help with the if the refresh doesn't come back in time you can try over and over, within the time limit, to get a success?
Guessing this will need updating ...
https://github.com/aarondcoleman/Fitbit.NET/blob/async-portable/Fitbit.Portable/OAuth2/OAuth2AutoRefreshInterceptor.cs#L18
.... with a change ...
https://github.com/aarondcoleman/Fitbit.NET/blob/async-portable/Fitbit.Portable/FitbitClient.cs#L156
... which I'm not a fan of. It doesn't feel right for it to be a method on the FitbitClient directly, but happy to leave if it fits with peoples usages :-)
And here ...
https://github.com/aarondcoleman/Fitbit.NET/blob/async-portable/Fitbit.Portable/OAuth2/DefaultTokenManager.cs#L24
... as I'm guessing this is where the "retry" logic will be?